Abstract: Microwave equivalent circuit (microwave net) is adopt to study the rectangle wave-guide output window of klystron. Transfer matrix is utilized to calculate the standing wave ratio and optimal size of the window is found based on the condition of no reflection. Meanwhile, a more precise simulation is done by ISFEL3D. Compared the result by microwave equivalent circuit method to the result by ISFEL3D and experiment, the conclusion indicates that the method is feasible for the initial design, which is easy to programe and fast to compute, without much distinction to experimental result, based on its brief theory.